Breakdown of adiabatic invariance in spherical tokamaks

Due to the weak but strongly varying magnetic field, fast ions in spherical tokamaks are susceptible to the breakdown of adiabatic invariance. Beam ions in present-day spherical tokamaks are typically marginally adiabatic or super-adiabatic. Super-adiabatic ions have energies in the intermediate range where the longitudinal invariant is still adiabatically invariant, but the magnetic moment is not, and their motion is integrable. Fusion alphas in proposed burning-plasma experiments would, however, to a large extent be nonadiabatic, and follow chaotic orbits.